Inhibin concentrations in mares with granulosa cell tumors

Abstract The hormone-producing equine granulosa cell tumor (GCT) may secrete high levels of inhibin. Measurement of inhibin concentrations may be useful in the diagnosis and conformation of mares with GCT. Inhibin may be measured using RIA, which recognizes dimeric αβ A -inhibin as well as the monomeric (free) inhibin α-subunit, or using a two-site immunoradiometric assay (IRMA) specific for αβ A -inhibin. The objective of this study was to examine concurrent relationships among α-inhibin (as measured using RIA), αβ A -inhibin (as measured using IRMA), and other hormones (testosterone, estradiol, LH, FSH) in mares with GCT. Hormone concentrations were measured in single serum or plasma samples obtained from 22 mares with GCT and from 31 normal cycling mares. One GCT mare had blood samples collected at 12-h intervals for 21 days, and at 15-min intervals for two 6-h periods during that time. Results showed that in GCT mares α-inhibin was increased to a greater extent, was more uniformly elevated, and had a less variable secretory pattern than did αβ A -inhibin. Concentrations of α-inhibin and tumor mass were positively correlated ( P P P =0.05). Concentrations of FSH tended to be lower in GCT than control mares and were inversely related with αβ A -inhibin in GCT mares. Testosterone and estradiol concentrations were variable. It was concluded that immunoreactive α-inhibin reflected detection of both αβ A -inhibin and free α-subunit. Free α-subunit was evidently secreted at a relatively steady rate dependent upon mass of the GCT, whereas secretion of αβ A -inhibin was more responsive to FSH regulation. Determination of α-inhibin using RIA appeared to be a more reliable indicator of the presence of a GCT than specific measurement of αβ A -inhibin using IRMA.